The present invention relates in general to microwave signal processing circuitry and, more particularly, to a microwave filter illustrated in microstrip technology in which it is initially being used.
In microwave circuit design, a so-called “notch filter” can be used to reject a specific frequency range, but allow other frequencies to pass with low loss. The notch filter usually features a sharp notch in its frequency response curve with the notch substantially spanning the rejected frequency range of interest. A known microstrip structure for a notch filter 100 is shown in FIG. 1 wherein microwave energy passes along a microstrip transmission line 102 of the filter 100 from an input end or port 104 to an output end or port 106. An open stub 108, having a nominal electrical length of ¼λ where λ is the wavelength of the desired notch frequency (i.e., the central frequency of the band to be rejected), is connected to the transmission line 102 as shown. One characteristic of an open stub, such as the stub 108 shown in FIG. 1, is that energy at the notch frequency and its odd harmonics is rejected or attenuated as it passes through the transmission line 102. However, energy at frequencies below and in between the odd harmonics of the notch frequency are passed through the transmission line 102, see FIG. 2.
Multiple stubs may be cascaded to enhance the frequency response, for example as shown in FIG. 3, two stubs 108a, 108b are spaced apart on the transmission line 102. The physical dimensions and relative placement of the stubs 108a, 108b with respect to the transmission line 102 can be used to adjust the input impedance seen by a source at the input port 104 and thereby may be used to frequency tailor a desired filter response. It can be seen from the frequency response 400 in FIG. 4 that a filter 300 having multiple stubs can be used to extend the depth of the notches (over 55 dB shown in FIG. 4 in comparison to less than 30 dB shown in FIG. 2) and the symmetry of the frequency response. In all cases, however, the odd harmonics or odd multiples of the notch frequency are also suppressed. For example, suppression of the third harmonic appears in both FIGS. 2 and 4.
Another known notch filter circuit 500 shown in FIG. 5 illustrates what is referred to as a “spurline” structure. A spurline 501 is formed by removing an L-shaped portion of a microstrip 502, with one end of the spurline 501 open to one side of the microstrip 502 and the remainder extending along and contained within the microstrip 502. Microwave energy is fed into the microstrip 502 at an input end or port 504 and exits at an output end or port 506. Again, the spurline 501 has a nominal length of ¼ of the wavelength of the desired notch frequency, i.e., a nominal length of ¼λ. As shown in FIG. 6, the filter circuit 500 effectively rejects a desired notch frequency and its associated odd harmonics (only the third harmonic shown). Due to edge effects and other considerations, the frequencies of the rejected harmonics may not be exactly 3, 5, 7 or other odd multiples of the fundamental notch frequency as is known in the art and reflected in FIGS. 2, 4 and 6.
In some applications, it may be desired to reject a specific notch frequency but not the associated third harmonic. In other applications, it may be desired to reject a specific frequency but not at least one specific higher order odd harmonic, such as the 5th, 7th, 9th or other higher order odd harmonic.
In order to reject a specific notch frequency but accept the third harmonic, or some other higher order odd harmonic, normally a more complex circuit is required, such as a combination of cascaded high-pass and low-pass circuits. A more complex circuit in turn demands more space on the circuit board, and entails more loss due to the ohmic losses in the conductor making up the circuit.